The present invention relates generally to trunked radio communication systems employing vehicular repeaters.
Municipal agencies commonly employ public service trunked (PST) radio services to provide communication between various individuals within a metropolitan area, such as the employees of a city's fire and police departments. Private entrepreneurs may also provide trunked radio systems at one or more sites within a given geographical area for use by one or more independent business entities using special mobile radios (SMR). Typically, the users within these systems share a prescribed number of frequency channels. Access to the channels is coordinated by a central trunking processor.
In recent years, trunked radio communication systems have employed digital data protocols (including the use of digital control channels) to provide a number of special features. For instance, the use of a digital data protocol enables a central dispatcher to selectively transmit a message to an individual's mobile radio. The dispatcher may also transmit a message to a predefined group of mobile radios, such that each member within the group simultaneously receives the message. For instance, a typical PST system might allocate groups (or fleets) corresponding to squad car patrolmen, foot patrolmen, narcotics officers, etc. A discussion of various special features typically provided in trunked radio communication systems may be found in commonly assigned U.S. Pat. No. 4,905,302 to Childress et al., which is incorporated herein in its entirety.
In such systems it is generally desirable to provide "balanced" mobile radio coverage. A system is "balanced" when each mobile radio that receives messages from a given base site may, in turn, successfully transmit messages to the base site. This goal may be achieved to some extent by appropriately adjusting the location and output power of the base site equipment, as well as the output power of the mobile radios.
However, in many applications it is necessary to communicate between a base site and one or more hand-held portable radios. For instance, foot patrolmen typically carry small portable radios to communicate with a central base site. Also, squad car officers typically carry similar portable radios when leaving their vehicles to investigate an accident, complaint or disturbance. Because of size constraints, these smaller mobile radio units often do not have sufficient power to reach a central base site, thereby preventing balanced coverage.
System designers have addressed this problem through the use of repeaters. In operation, the low power portable stations transmit their messages to a repeater station, where they are forwarded to the base site, typically at a higher power level. The repeater station may be mobile. For instance, the messages transmitted by a police officer's portable radio might be transmitted to a repeater housed in the officer's squad car, and then forward to the base site. Commonly assigned U.S. Pat. No. 4,150,334 to Williams exemplifies this technique.
Typically, vehicular repeaters employed a first band of frequencies when communicating with the portable radios, and a second separate band of frequencies when communicating with the base site equipment. When the vehicular repeater received a message from a portable radio, it converted the message to the band of frequencies compatible with the base site equipment. This technique is exemplified in U.S. Pat. No. 4,553,262 to Coe, assigned to Motorola, Inc. In this manner, the system designer could be assured that the vehicular repeaters would not interfere with the operation of the base site equipment.
Although conceptually simple, this solution suffers many drawbacks. Notably, the portable radio was limited to use with the repeater. Even if the portable station had sufficient power to successfully transmit a message to the base site equipment, it could not do so, as the base site equipment operated using a different communication band.
Furthermore, these portable radios and their associated repeaters were typically configured to communicate with each other on a single frequency in a non-trunked (or "conventional") manner separate from the digital protocol employed by the base station and associated trunked mobile receivers. Because of these limitations, various special services provided by the digital trunking system did not extend to the portable radios. These services include, but are not limited to, digital identification of individual and groups of mobiles, emergency and/or priority channel access, wide area "multi-site" interconnections via digital switches, and communication security through voice encryption. In addition to physical signalling limitations, the FCC prohibits trunked operation in certain communication bands commonly used for the "conventional" vehicular repeaters.
Also, because of their single-frequency non-trunked mode of operation, "conventional" radio repeater systems could not accommodate two or more repeaters operating at the same time within close proximity to one another. Otherwise, the two or more repeaters would interfere with each other. For this reason, typical repeater-based radio communication systems assigned one of the repeaters (the "priority repeater") the role of relaying all communication between the portable units and the base station. This had the drawback of overworking the assigned priority repeater and thereby depleting its battery supply. Schemes were devised to shift the role of priority repeater from one repeater to another according to various algorithms. However, these algorithms sometimes delayed communication routed through the repeater.
Finally, available radio spectrum for trunked systems has become a scarce commodity in many areas. Many times, insufficient spectrum exists to allocate a separate band of frequencies to handle communication between the portable radios and the vehicular repeaters.
Accordingly, it is an objective of the invention to provide a vehicular repeater system which does not suffer from the above described drawbacks.
It is a more particular objective of the present invention to provide a vehicular repeater system which makes efficient use of available channels, and which extends the special features provided by a trunked radio systems to portable radios communicating with a base station through a repeater.